Marine diesel engines in which the utility turbine is used for generating the electrical energy required on the ship are known.
Thus, for example, a generator can be coupled directly to the reduction gearing of the high-speed utility turbine. A unit of this type has the advantage that it can be accommodated somewhere in the freely available space of the ship's engine room provided the gas lines are not too long. The disadvantage of this solution is in the need for elaborate power electronics with which the turbine/generator unit is to be regulated to a specified speed. The fact that controls of this type cost several times the price of the turbine eliminates them from the restricted choice. Using economical asynchronous generators, which is actually the most obvious solution, is also ruled out, because, in marine installations, in contrast to stationary installations--in which a generally large electrical power system keeps the turbine/generator unit at a constant speed--the entire output of the ship's electrical system is approximately of the order of magnitude of the utility turbine output; the ship's electrical system is therefore unable to keep the speed of a utility turbine/asynchronous generator constant.
No more useful is a further method of speed control in which a synchronous generator is coupled to the utility turbine and this unit is regulated by means of valves in the exhaust gas. The problem here, in particular, is the costly arrangement of controllable valves in the exhaust gas flow of installations run on heavy oil, and also the dynamic processes in the long exhaust-gas lines, which processes can be difficult to monitor.
Another possibility of using the utility turbine for the generation of current lies in marine diesel engines which themselves drive a speed-controlled generator. Here, the utility turbine can be coupled to the free shaft end of this generator. However, as a result of the costly speed control, many marine engines are not equipped with a separate generator. As a rule, provision is then made for the utility turbine output to be delivered to the crankshaft of the main engine via likewise costly gearing.
A supercharged marine diesel engine of the type mentioned at the beginning (that is, in which the utility turbine is coupled to a free shaft end of the auxiliary diesel/generator unit) is described in the journal "HANSA-Schiffahrt-Schiffbau-Hafen", 122nd year--1985, No. 22, pages 2304-2310. This solution, designated as a complete integrated system, has the advantage that the power requirement can be met in the most simple, inexpensive and reliable manner, with it being possible to dispense with all of the abovementioned auxiliary generators, gearing and control devices.
However, when a turbocharged diesel engine is at low load (the pressure difference across the engine, that is, the difference between the pressures in the exhaust manifold and in the air receiver) is very small. It can even be sometimes negative. Since, however, reciprocating engines always have a certain valve overlap (that is, phases in which the inlet valves and the exhaust valves are open at the same time) combustion gases can flow back into the inlet ports when there is a negative pressure difference across the engine. The inlet ports can become severely contaminated, especially in heavy-oil operation, in which the combustion gases contain many particles of dirt.